Mature 5; termini of transfer RNAs (tRNAs) in both prokaryotes and eukaryotes are generated from precursor molecules by cleavage with a structure-specific endonuclease, RNase P. The enzyme contains both RNA and protein components, and the RNA component alone from either E. coli or B. subtilis can catalyze the correct cleavage. Further, the RNA and protein components from these two prokaryotes can e interchange to reconstitute RNase P holoenzymes even though the primary sequences of the two RNAs show little homology. Cleavage appears to precede through a mechanism distinct from those observed in RNA splicing reactions, as judged by cofactor requirements and the dependence on substrate structure, rather than sequence. Two forms of RNase P have been identified in the yeast S. cerevisiae; a nuclear form and a mitochondrial form from which only an RNA component is encoded in the mitochondrial DNA. We have isolated the nuclear form and sequenced the RNA component. This proposal details plans to: 1) characterize the RNase P RNA gene, 3) determine the solution structure of the RNase P holoenzyme and enzyme-substrate complex, 3) examine which features of the enzymic and substrate RNA structures are required for correct cleavage, 4) investigate the structure of the active site and mechanism of catalysis and 5) determine whether the mitochondrial and nuclear enzymic RNAs share the same protein component(s).